What is the angle formed by a horizontal line and a line of sight to a point below?

You are describing the "angle of depression". No, this term isn't a new flavor of mental illness. It's actually the term that is used to describe the angle between the horizontal line and your line of vision toward an object below you.

Let's say that you have climbed a tree at the local park. While you're enjoying the pleasant weather from the tree, you notice a squirrel run towards the tree you've climbed and stop a few feet away. You and the squirrel make eye contact and begin a fierce stare down battle (the winner gets to be in the tree). This stare down is an example of an angle of depression.

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The line of sight (in our example, the stare down line between you and the squirrel) creates an angle of depression with the horizontal line (the imaginary line that starts at your eyes and moves out horizontally, parallel to the ground). The angle of depression is congruent, or is identical, to the angle of elevation to the squirrel. This is considered an angle of elevation because the object the squirrel is looking at is above his line of sight. So why don't we ride up in an Elevator and ride down in a Depressor, you might ask? That will have to be the topic of another Ask Cliff discussion.